When comparing SciTE vs FlashDevelop, the Slant community recommends SciTE for most people. In the question“What are the best programming text editors?”SciTE is ranked 13th while FlashDevelop is ranked 34th. The most important reason people chose SciTE is:

Based on the Scintilla source code editor, SciTE has some advanced features like rectangular editing, simple regular expression search and replace, code folding, etc. It allows the user to launch a compiler or interpreter, and it can also interpret the error messages, jumping at the location they point to.
Lua scripting is key to SciTE's power and flexibility. The Lua scripting language can be used to perform complex text transformations. It's relatively simple syntax and its large user-base makes it a great choice for a scripting feature.

Pros

Pro

Powerful

Based on the Scintilla source code editor, SciTE has some advanced features like rectangular editing, simple regular expression search and replace, code folding, etc. It allows the user to launch a compiler or interpreter, and it can also interpret the error messages, jumping at the location they point to.

Lua scripting is key to SciTE's power and flexibility. The Lua scripting language can be used to perform complex text transformations. It's relatively simple syntax and its large user-base makes it a great choice for a scripting feature.

Pro

Flexible

It's property files allow for fine tweaks of its behavior, at a global or per language / project level. These textual settings might be confusing for those used to preference dialogs, but prove to be powerful, flexible, and fine grained.

Pro

Lightweight

With less than 2 MB of binary on Windows, SciTE starts instantly. Plus, if you don't need all the config, syntax files, blah, there's a 678k standalone .exe version. Nothing is going to beat that for lightweight and start-up times. Stick it in a folder that is already on your PATH.

Pro

Built-in shell

The console window can show the result of ran commands (like build current file, reporting warnings, and errors), but also accept interactive shell commands.

Pro

Portable

SciTE works on Windows and Linux, and it also has a commercial port on MacOS.

Pro

Powerful syntax highlighting for numerous languages

Lexers providing folding and syntax highlighting are based on code, not on regular expressions. They support context, nesting, special rules, etc.

Pro

Free (except on Mac) and open source

SciTE is written in C++, with lot of contributors, both to the core and to the numerous lexers.

Pro

GUI

Has a simple graphical user interface

Pro

Free

FlashDevelop comes without any cost.

Pro

Good code completion

FlashDevelop's code completion is pretty good.

Pro

Haxe development support

FlashDevelop has first-class support for Haxe development, the open source toolkit based on a modern programming language and cross-platform library.

Pro

Excellent support for Actionscript 2 and 3 (Flash)

Although everyone claims Flash is dead, it's still quite useful for game developers due to its rapid compile and run times, as well as its great debugging functionality with FlashDevelop.

Pro

Good number of project templates

While it's project template system is not the best compared to it's competitors, it still is decent and is a good way to generate some boilerplate code.

Pro

XML/HTML completion

FlashDevelop has XML/HTML completion aside from code completion.

Pro

.NET Framework 2.0 application

It's windows only, but has tremendous support from plugin developers and a dedicated team that's been developing it for close to 10 years.

Pro

Source-control support (svn, git, mercurial)

Pro

Great debugging

FlashDevelop provides very efficient debugging features.

Pro

Supports Zen-coding for HTML

This is very useful for carrying out high-speed HTML coding and editing.

Pro

Snippets

Pro

Tasks/todo

Pro

SWF/SWC exploration

Pro

Great project compilation

FlashDevelop facilitates project compilation.

Pro

Decent code generation

Although the code generation can't really be called top-notch, it's decent and sufficient for most developers.

Cons

Con

Hard to config

The configuration is mainly a file-based config, which can be unintuitive and difficult to use for new users.

Con

Missing file browser

SciTE's greatest weakness is perhaps the file browser. It does not really have one, just a poor substitute which works a little bit like a terminal window with ls or dir commands to show the files in a directory.